Feeding of the 1$^{-}$ isomer in $^{176}$Lu in neutron induced reactions

The ($n,n^{\prime}\gamma$) reaction was used to measure $\gamma$-ray excitation functions in $^{175,176}$Lu with a $^{nat}$Lu target. The data were taken using the GEANIE spectrometer comprised of 26 Ge detectors. The pulsed neutron source of the LANSCE/WNR facility provided neutrons with incident energies from 1 to 300~MeV, determined using the time-of-flight technique. The excitation functions for 111 $\gamma$-rays from several reaction channels were determined, among them two transitions feeding directly the 1$^{-}$, 3.664h isomer, and one transition feeding directly the 7$^{-}$, 3.76${\times}10^{10}y$ ground state in $^{176}$Lu. Both states $\beta$-decay to $^{176}$Hf. The Lu-Hf radioactive decay system, with a half-life of $\sim$37 billion years from the ground state, is a chronometer for dynamic processes on a stellar time scale. Recent anomalies have questioned the accuracy of this chronometer, one possible explanation being that astrophysical ($n,n^{\prime}$) reactions populate the 1$^{-}$ isomer and reduce the effective half-life of $^{176}$Lu. The excitation functions for the three $^{176}$Lu transitions from the present experiment can be compared to nuclear reaction model calculations that predict the $^{176g}$Lu($n,n^{\prime}\gamma$)$^{176m}$Lu cross section, thus providing an important check for such models. Attempts to improve the experimental results to obtain absolute partial $\gamma$-ray cross sections for more $\gamma$-rays feeding the isomer and ground state of $^{176}$Lu in future GEANIE experiments will be discussed. This work was supported by U.S. DOE DE-AC52-06NA25396.